Gas-liquid contactors and absorbers are widely used to remove substances such as gases and particulate matter from combustion or flue gases produced by utility and industrial plants. Often of particular concern are sulfur dioxide (SO.sub.2) and other acidic gases produced by the combustion of fossil fuels and various industrial operations. Such gases are known to be hazardous to the environment, and their emission into the atmosphere is closely regulated by clean air statutes. The method by which these gases are removed with a gas-liquid contactor or absorber is known as wet flue gas desulfurization.
The cleansing action produced by a gas-liquid contactor is generally derived from the passage of gas through a tower cocurrently or countercurrently to a descending liquid that cleans the gas. Wet flue gas desulfurization processes typically involve the use of calcium-based slurries or sodium-based or ammonia-based solutions. Examples of calcium-based slurries are limestone (calcium carbonate; CaCO.sub.3) slurries and hydrated lime (calcium hydroxide; Ca(OH).sub.2) slurries formed by action of water on lime (calcium oxide; CaO). Such slurries react with the acidic gases to form precipitates that can be collected for disposal or recycling. Intimate contact between the alkaline slurry and acidic gases that are present in the flue gases, such as sulfur dioxide, hydrogen chloride (HCl) and hydrogen fluoride (HF), result in the absorption of the gases by the slurry and the formation of salts, such as calcium sulfite (CaSO.sub.3 .multidot. 1/2H.sub.2 O), gypsum (CaSO.sub.4 .multidot.2H.sub.2 O), calcium chloride (CaCl.sub.2) and calcium fluoride (CaF.sub.2). Forced oxidation of the slurry by aeration is often employed to ensure that all of the sulfites will be reacted to form sulfates, and thereby maximize the production of gypsum.
While gas-liquid contactors and absorbers utilizing calcium-based slurries as described above generally perform satisfactorily, their operation results in the production of large quantities of wastes or gypsum, the latter having only nominal commercial value. In contrast, ammonia-based scrubbing processes have been used in the art to produce a more valuable ammonium sulfate fertilizer. In these processes, sulfur dioxide from the flue gases reacts with ammonia to form an ammonium sulfate solution or ammonium sulfate crystals ((NH.sub.4).sub.2 SO.sub.4). A particular example of such a process is disclosed in U.S. Pat. No. 5,362,458, assigned to the assignee of the present invention, and results in the production of ammonium sulfate fertilizer. However, in many markets, the added value of ammonium sulfate over the value of ammonia is minimal. In addition, such prior art processes have required bulk supplies of ammonia that are consumed by the desulfurization process, necessitating the transportation and on-site storage of large quantities of ammonia. Because transportation and storage of ammonia are highly regulated and relatively costly, the production of ammonium sulfate using flue gas desulfurization systems is typically suitable for niche markets only.
From the above, it can be appreciated that prior art desulfurization processes generally have the disadvantage of producing, at best, a byproduct having only nominal market value, and therefore only a minor effect on the economic aspects of the desulfurization process. Accordingly, it would be desirable if a flue gas desulfurization process produced a product more valuable than the gypsum produced using calcium-based slurries, and was more economical than prior an methods employed in the production of ammonium sulfate fertilizer using ammonia-based solutions.